(This only works because there already exists a hash reference at $ipairs{$fk}{$pair}. It would not work if you tried to say $pvt = {}, but %$pvt = () would. It's all reference magic and not really easy to explain unless you first understand pointers.)

(Of course, the hash slice can be rewritten using a temporary variable, too. It's always a good idea to use temporary variables if it makes your code easier to understand. Triply a good idea if it reduces repetition.)